With the advancement of shape control actuators and the development of new shape control technologies, the shape quality of cold rolled strip has improved significantly in recent years. With the use of shape meters and closed loop controls, simple shape defects, such as edge waves and center waves, are no longer difficult to control. With the reduction of simple shape defects, the focus has turned to local shape defects. In many cases, local shape defects are the number one reason for customer rejection of cold rolled strip. Unlike simple waves, local shape defects are, usually related to abnormal rolling conditions, such as feed stock ridges yield stress local drop, improper roll cooling practice and roll cooling nozzle clogging etc., and are therefore difficult to control by the use of any existing shape control actuators in a cold mill (Liu et al, Three-dimensional Simulation of Local shape Defects in Continuous Cold Rolling, Iron & Steel Technology, August 2007, p 70-80). Although local shape defects may be caused by a variety of abnormal rolling conditions, the majority of local shape defects are caused by feed stock ridges (Melfo et al, Ridge-buckle Defect in Thin-rolled Steel Strip, Iron and Steel Technology, August 2006, p. 54-61) which are areas of local thickening in the feed stock as shown in FIG. 1. FIG. 1 is a transverse cross-section of a small portion of one surface of a feedstock strip 2, which is typically a hot rolled strip, having a single ridge 4. As seen in FIG. 1, the surface of feedstock 2 is uneven with the heavy line 6 denoting the nominal thickness of the feedstock.
Examples of local shape defects caused by feed stock ridges are ridge-buckles 8 as shown in FIG. 2 (Melfo et al, 2006 mentioned above). FIG. 2 shows a coil of cold rolled strip having ridge-buckles 8 and a characteristic build-up 10. Ridge-buckle defects typically consist of sinusoidal waveforms between 100 and 200 mm in width and up to around 3 mm in amplitude, and with a pitch of about 200-300 mm that appear in thin strip following cold rolling, annealing and tempering. The characteristic build-up 10 is basically caused by ridges remaining in the cold rolled strip. The remaining thickness ridges will induce high localized pressure between laps of the coil, which often cause the laps to stick together in subsequent batch annealing process, which defect is called annealing sticker.
The control and reduction of ridge-buckle defects and annealing stickers have been leading research topics in the field of flat rolling as existing shape control actuators cannot effectively control ridge-buckles (Melfo et al, 2006 mentioned above) and annealing stickers consistently cause large losses. So far, an effective solution for decreasing or eliminating the incidence of ridge-buckles had not been found.